1. Field of the Invention
The present invention relates to an optical apparatus, and more specifically to a zoom lens applicable for use in an optical apparatus such as a digital still camera, a video camera, or a TV camera.
2. Description of the Related Art
Image pickup apparatuses (cameras), such as a video camera and a digital still camera, using a solid-state image pickup element have steadily increased in functionality and decreased in size. As an optical system for use in such image pickup apparatuses, there is a demand for a compact zoom lens having a wide viewing angle, high aperture ratio, and high optical performance.
In a camera using a solid-state image pickup element, a zoom lens should preferably have a relatively long back focus because various optical components, such as a low-pass filter and a color correction filter, are disposed between the rear of the zoom lens and the solid-state image pickup element. In addition, a camera including a solid-state image pickup element for a color image is required to include a zoom lens having a good image-side telecentric characteristic in order to avoid color shading.
A compact zoom lens having a long back focus and a good image-side telecentric characteristic is known. One example is a negative lead type zoom lens in which a lens unit having a negative refractive power is located closest to an object side. More specifically, for example, U.S. Pat. No. 7,023,623 and Japanese Patent Laid-Open No. 2006-208890 disclose three-unit zoom lenses that include, in order from an object side to an image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power and that has a good image-side telecentric characteristic.
In a negative lead type zoom lens including three or more lens units, it is effective to increase the refractive powers of the lens units in order to reduce the total size of the zoom lens and to increase the zoom ratio.
However, if the refractive powers of the lens units are simply increased for a wider viewing angle and a higher zoom ratio, aberration variation during zooming increases. Therefore, it is difficult to achieve high optical performance over the entire zoom range of known negative lead type zoom lenses. Accordingly, to achieve a compact total size, a wide viewing angle, and a high zoom ratio in the negative lead type zoom lens including three or more lens units, it is important to appropriately set the refractive powers of the lens units and moving paths and moving amounts of the lens units during zooming.
For example, if an attempt to reduce the size of the total lens system and to increase the viewing angle is made without appropriately setting the lateral magnification of the second lens unit and the refractive powers of the first and third lens units, optical performance at the wide angle end can be seriously affected in a negative manner. Further, if an attempt to increase the viewing angle and zoom ratio is made without appropriately setting the moving path and moving amount of the first lens unit during zooming, the effective diameter of the lens closest to the object side increases, and this in turn increases the total size of the zoom lens.